1983 ford f250 van

my friends van was starting up fine with no problems then one day the starter started to slip so we changed the starter and it still continued to do the same thing then we changed the flywheel because it did have some of the teeth broken we thought that we had done it but then a couple of days later it started doing the samething we checked the starter agin and the flywheel also it is ok what can it be thats causing this problem


any help that you can give me is greatly appriciated